This invention relates to the storage system for water purified by reverse osmosis. More particularly, the invention relates to a bladder containing a purified water tank having a chamber exterior of the bladder for retaining a pressurized fluid, preferably air. The volume of the chamber exterior to the bladder is approximately equal to the volume of the bladder when the bladder is full.
The process of purifying hard or brackish water by reverse osmosis is well known and is in wide commercial usage. Typically, brackish water under pressure is passed over a thin reverse osmosis membrane, commonly made from cellulose acetate; water passes through the membrane and the majority of impurities are rejected at the surface and swept away. Water passing through the membrane is collected in a storage tank for use upon demand. Most commercial units are relatively small, being used primarily to provide drinking water in areas where municipal or well water contains large quantities of salts or impurities. Generally, these units are limited in capacity to a few gallons per day. Since the passage of water through the membrane is a very slow process, the purified water is made continuously and is stored in a tank until needed. The most common method of storing water is in a bladder tank, wherein the purified water passes from the reverse osmosis cartridge directly to the interior of a flexible bladder which is contained in a pressure tank. In order to maintain a driving force to move the purified water out of the tank to a purified water faucet upon demand, the interior of the tank outside of the bladder is generally pressurized with about 5 psi of air. Accordingly, as the bladder fills with purified water, the bladder expands within the tank and the pressure within the tank increases gradually from 5 psig to the line pressure upstream of the reverse osmosis unit. When the bladder is full, the pressure in the bladder equals line pressure, and flow through the reverse osmosis membrane stops since there is no pressure drop across the membrane.
Two undesirable events occur as the bladder fills and the pressure drop across the membrane decreases. First, the volume of flow through the membrane is directly related to the pressure drop across the membrane. Therefore, as the pressure drop decreases, the volume of flow through the membrane decreases substantially. More importantly, however, it has been found that the efficiency of a reverse osmosis membrane, particularly a cellulose acetate membrane, is very highly dependent on the pressure drop across the membrane. For example, it has been found that at a pressure drop across a cellulose acetate membrane of about 25 psi, nearly 90% of the salts are rejected by the membrane. At a pressure drop of 10 psi across the membrane, however, only about a 60% rejection can be expected.
Accordingly, as the bladder fills from an entirely empty state, the pressure drop across the membrane will decrease from an initial pressure drop equal to line pressure minus tank pressure (i.e., line pressure--5 psi), to zero. Since the pressure drop is large when the tank is empty, water made as the tank is just beginning to fill can be expected to have a high degree of purity. When the pressure drop declines to below approximately 25 psi, the quality of water passing the membrane becomes increasingly poor. In actual usage, this problem becomes particularly aggrevated by the habits of most users to draw relatively small quantities of water from the tank at a time. For example, if a tank has filled to its capacity (e.g., 3 gallons) overnight, and a user draws off a glass of water to drink, the withdrawal of the glass of water may only generate a pressure drop of a few psi across the membrane. Repeated daily withdrawals of relatively small quantities of water means that the membrane will be continually operating at a relatively low pressure drop, resulting in the continuous manufacture of water of relatively poor quality.
Accordingly, it is an object of the invention to provide a system for containing reverse osmosis water such that withdrawals of relatively small amounts of water, even from a full system, generate a substantial pressure drop across the reverse osmosis membrane. It is another object of the invention to provide a water storage system which enables the maintenance of a substantial pressure drop across the membrane, and which is easy to maintain and does not have a substantial number of working parts. It is a still further object of the invention to provide such a system which is relatively inexpensive, simple to install, and easy to maintain. These and other objects of the invention are accomplished by the use of the reverse osmosis water supply system as described herein.